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The present invention provides a zirconia-based mixed oxide which,
together with improving the heat resistance of specific surface area at a
high temperature (1000.degree. C. for 3 hours), has a ceria reduction
rate of 80% or more, or in other words, improves the heat resistance of
specific surface area and the reduction rate of ceria. The zirconia-based
mixed oxide has zirconia for the main component thereof and contains 5%
or more of ceria and 1 to 30% of a rare earth metal oxide other than
ceria, wherein the specific surface area after heat treating for 3 hours
at 1000.degree. C. is 50 m.sup.2/g or more, the reduction rate of the
ceria contained in the mixed oxide is 80% or more, and preferably the
specific surface area after heat treating for 3 hours at 1100.degree. C.
is 20 m.sup.2/g or more.

1. A zirconia-based mixed oxide comprising 1) zirconia for the main
component, 2) 5 wt % or more of ceria and 3) 1 to 30 wt % of a rare earth
metal oxide other than ceria, whereinthe specific surface area after heat
treating for 3 hours at 1000.degree. C. is 50 m.sup.2/g or more, and the
reduction rate of the ceria in the mixed oxide is 80% or more.

2. The zirconia-based mixed oxide according to claim 1, wherein the
specific surface area after heat treating for 3 hours at 1100.degree. C.
is 20 m.sup.2/g or more.

3. The zirconia-based mixed oxide according to claim 1 or 2, wherein the
rare earth metal oxide other than ceria includes at least one of an oxide
of lanthanum and an oxide of neodymium.

4. A method for producing a zirconium-based mixed oxide, comprising the
steps of:(1) mixing a zirconium salt with a salt of a rare earth metal
other than ceria in a solvent to obtain a solution containing zirconium
and a rare earth other than cerium;(2) adding an alkali to the solution
to obtain a mixed hydroxide containing zirconium and a rare earth other
than cerium;(3) dispersing the mixed hydroxide in water to obtain a
slurry followed by adding a cerium salt to the slurry;(4) adding an
alkali to the cerium salt added slurry to obtain a mixed hydroxide
containing zirconium, a rare earth other than cerium and cerium; and(5)
heat-treating the mixed hydroxide to obtain a mixed oxide comprising
zirconia, a rare earth metal oxide other than ceria and ceria.

5. The method for producing a zirconia-based mixed oxide according to
claim 4, wherein the salt of a rare earth metal other than cerium
includes at least one of a salt of lanthanum and a salt of neodymium.

Description

FIELD OF THE INVENTION

[0001]The present invention relates to a zirconia-based mixed oxide and a
production process thereof.

BACKGROUND OF THE INVENTION

[0002]The specific surface area of zirconia units conventionally used as
catalyst supports is at most about 100 m.sup.2/g at 400.degree. C. In
addition, those having a greater specific surface area are typically
amorphous without having a crystal structure. Consequently, even if a
zirconia unit is used as a catalyst support, as a result of the specific
surface area decreasing at high temperatures of 400.degree. C. or higher,
it is not possible to obtain stable performance at high temperatures.
Thus, it is necessary to further improve heat resistance in order to use
as a catalyst support.

[0003]In contrast, zirconia-ceria compositions composed of zirconium oxide
and cerium oxide are typically able to secure a comparatively large
specific surface area even at a high temperature of 1000.degree. C., and
have heat resistance superior to that of zirconia and the like when used
as a catalyst.

[0004]At present, there have been numerous reports of attempts to further
improve heat resistance by adding rare earth metal oxides or alkaline
earth metal oxides and the like other than ceria to zirconia-ceria
compositions.

[0005]In actuality, however, since the important function of a
co-catalyst, in addition to heat resistance, is the oxidation-reduction
potential of ceria in an oxidation-reduction atmosphere, it is becoming
an indispensable characteristic for improving catalyst performance.

[0006]Japanese Patent No. 3490456 describes a "composition having for a
base material thereof a zirconium oxide containing cerium oxide and at
least one type of doping element; wherein, the composition is provided in
the form of a single phase of zirconium oxide crystallized into a cubic
system or tetragonal system, the cerium oxide and doping element
contained therein is present as a solid solution, and the composition has
a specific surface area of 25 to 51 m.sup.2/g after firing for 6 hours at
1000.degree. C.".

[0007]In addition, Japanese Patent Application Publication No. H10-194742
describes a "zirconium-cerium-based mixed oxide obtained by firing at 500
to 1000.degree. C.; wherein, the mixed oxide contains zirconium and
cerium, the mixing ratio of the zirconium and cerium as zirconium oxide
and cerium (IV) oxide is 51 to 95:49 to 5 as the weigh ratio thereof, the
mixed oxide demonstrates a specific surface area after the firing for 6
hours at 500 to 1000.degree. C. of at least 50 m.sup.2/g, and maintains a
specific surface area of at least 20 m.sup.2/g after heating for 6 hours
at 1100.degree. C.".

[0008]However, there is no description regarding the reduction rate of
ceria in Japanese Patent No. 3490456 and Japanese Patent Application
Publication No. H10-194742.

[0009]On the other hand, Japanese Patent No. 3623517 describes a
"composition comprising cerium oxide and zirconium oxide of at least one
cerium/zirconium atomic ratio; wherein the composition demonstrates a
specific surface area of at least 35 m.sup.2/g after firing for 6 hours
at 900.degree. C., and demonstrates an oxygen storage capacity of 1.5
ml/g at 400.degree. C.".

[0010]However, the ceria reduction rate is described in the examples as
being a low value of a maximum of about 12%.

[0011]Moreover, Published Japanese Translation No. 2006-513973 of a PCT
International Publication describes a "composition containing zirconium
oxide and cerium oxide having a ratio of zirconium oxide of at least 50%
by weight; wherein, the maximum reductibility temperature after firing
for 6 hours at 500.degree. C. is 500.degree. C. or lower, the specific
surface area is 40 m.sup.2/g or more, and the composition is in the form
of a tetragonal system phase."

[0012]However, although reduction rate of ceria is described as being 80%
in the examples, the specific surface area of 38 m.sup.2/g after heat
treatment for 6 hours at 1000.degree. C. is not satisfactory in terms of
heat resistance.

SUMMARY OF THE INVENTION

[0013]With the foregoing in view, an object of the present invention is to
provide a zirconia-based compound oxide which, together with improving
the heat resistance of specific surface area at a high temperature
(1000.degree. C. for 3 hours), has a ceria reduction rate of 80% or more,
or in other words, improves the heat resistance of specific surface area
and the reduction rate of ceria.

[0014]As a result of conducting extensive studies to achieve the
above-mentioned object, the inventors of the present invention
unexpectedly found that a zirconia-based mixed oxide is obtained that
improves the heat resistance of specific surface area and the reduction
rate of ceria by fabricating a mixed hydroxide containing zirconium and a
rare earth other than cerium, followed by fabricating a compound
hydroxide containing zirconium, a rare earth other than cerium and cerium
that formed a cerium hydroxide layer on the outside thereof, and
subjecting to heat treatment.

[0015]The present invention provides the following on the basis of this
finding.

[0016]1. A zirconia-based mixed oxide comprising 1) zirconia for the main
component, 2) 5 wt % or more of ceria and 3) 1 to 30 wt % of a rare earth
metal oxide other than ceria, wherein

[0017]the specific surface area after heat treating for 3 hours at
1000.degree. C. is 50 m.sup.2/g or more, and the reduction rate of the
ceria in the mixed oxide is 80% or more.

[0018]2. The zirconia-based mixed oxide according to above 1, wherein the
specific surface area after heat treating for 3 hours at 1100.degree. C.
is 20 m.sup.2/g or more.

[0019]3. The zirconia-based mixed oxide according to above 1 or 2, wherein
the rare earth metal oxide other than ceria includes at least one of an
oxide of lanthanum and an oxide of neodymium.

[0020]4. A method for producing a zirconium-based mixed oxide, comprising
the steps of:

[0021](1) mixing a zirconium salt with a salt of a rare earth metal other
than ceria in a solvent to obtain a solution containing zirconium and a
rare earth other than cerium;

[0022](2) adding an alkali to the solution to obtain a mixed hydroxide
containing zirconium and a rare earth other than cerium;

[0023](3) dispersing the mixed hydroxide in water to obtain a slurry
followed by adding a cerium salt to the slurry;

[0024](4) adding an alkali to the cerium salt added slurry to obtain a
mixed hydroxide containing zirconium, a rare earth other than cerium and
cerium; and

[0025](5) heat-treating the mixed hydroxide to obtain a mixed oxide
comprising zirconia, a rare earth metal oxide other than ceria and ceria.

[0026]5. The method for producing a zirconia-based mixed oxide according
to above 4, wherein the salt of a rare earth metal other than cerium
includes at least one of a salt of lanthanum and a salt of neodymium.

ADVANTAGES OF THE INVENTION

[0027]According to the present invention, a zirconia-based compound oxide
which, together with improving the heat resistance of specific surface
area at a high temperature (1000.degree. C. for 3 hours), has a ceria
reduction rate of 80% or more, or in other words, improves the heat
resistance of specific surface area and the reduction rate of ceria, and
a simple production process thereof, can be provided, and can be
preferably used as a catalyst material for treating internal combustion
engine exhaust gas and the like in this field.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028]The following provides a detailed explanation of the zirconia-based
compound oxide of the present invention and a production process thereof.

[0029]Furthermore, the zirconia referred to in the present invention is a
typically zirconia containing no more than 10% by weight of impurity
metals, including hafnia.

1. Zirconia-Based Mixed Oxide

[0030]The zirconia-based compound oxide of the present invention is a
compound oxide comprising mainly of zirconia and containing 5 wt % or
more of ceria and 1 to 30 wt % of a rare earth metal oxide other than
ceria; wherein, the specific surface area following heat treatment at
1000.degree. C. for 3 hours is 50 m.sup.2/g or more, and the reduction
rate of the ceria contained in the mixed oxide is 80 wt % or more.

[0031]First, the composition comprises mainly of zirconia. More
specifically, the zirconia content is 50 to 90 wt % and preferably 55 to
85 wt %. If the zirconia content is less than 50 wt %, or more than 90 wt
%, the specific surface area following heat treatment at 1000.degree. C.
is less than 50 m.sup.2/g, thereby making this undesirable.

[0032]The ceria content is 5 wt % or more, and more specifically, 5 to 49
wt % and preferably 5 to 40 wt %. If the ceria content is less than 5 wt
%, the specific surface area following heat treatment at 1000.degree. C.
is less than 50 m.sup.2/g, while if the ceria content exceeds 49 wt %,
the ceria reduction rate is less than 80% and the reduction rate
decreases, thereby making this undesirable.

[0033]The content of rare earth metal oxide other than ceria is 1 to 30 wt
% and preferably 5 to 25 wt %. If the content of rare earth metal oxide
is less than 1 wt % or more than 30 wt %, the specific surface area
following heat treatment at 1000.degree. C. is less than 50 m.sup.2/g,
thereby making this undesirable.

[0034]Furthermore, examples of rare earth metal oxides other than ceria
include oxides of lanthanoid elements such as scandium, yttrium,
lanthanum, praseodymium and neodymium. Among these, at least one type of
oxide of either lanthanum or neodymium is preferably contained, and at
least lanthanum and neodymium are particularly preferably contained.

[0035]Next, the zirconia-based mixed oxide of the present invention has a
specific surface area following heat treatment for 3 hours at
1000.degree. C. of 50 m.sup.2/g or more, and preferably 55 m.sup.2/g or
more, and the reduction rate of ceria contained in the mixed oxide is 80%
or more and preferably 82% or more.

[0036]If the specific surface area following heat treatment for 3 hours at
1000.degree. C. is less than 50 m.sup.2/g, the rate of deterioration from
the initial specific surface area is high, sintering of platinum group
metal particles is accelerated by thermal contraction of the support,
thereby making this undesirable.

[0037]Furthermore, the reduction rate of the ceria is defined as 100% when
the total amount of ceria contained in the zirconia-based mixed oxide is
subjected to an oxidation-reduction reaction
(CeO.sub.21/2Ce.sub.2O.sub.3+1/4O.sub.2) (or using a different
expression, the reduction rate of ceria is defined as 100% when OSC
(oxygen storage capacity) is 0.25 mol-O.sub.2/mol-CeO.sub.2).

[0038]Moreover, the zirconia-based mixed oxide of the present invention
preferably has a specific surface area of 20 m.sup.2/g or more following
heat treatment for 3 hours at 1100.degree. C. If the specific surface
area following heat treatment for 3 hours at 1100.degree. C. is less than
20 m.sup.2/g, sintering of platinum group metal particles is accelerated
due to thermal contraction of the support, thereby making this
undesirable.

[0039]In this manner, together with improving the heat resistance of
specific surface area at a high temperature (1000.degree. C..times.3
hours), the zirconia-based mixed oxide of the present invention was
determined to improve the reduction rate of ceria to 80% or more, namely
improve the heat resistance of specific surface area and the reduction
rate of ceria.

[0040]This is thought to be because, in contrast to the surface
composition and bulk composition exhibiting identical values in the case
of conventional products, in the present invention, rare earth metals
other than cerium, preferably at least one type of either lanthanum or
neodymium, and particularly preferably at least lanthanum and neodymium,
are coprecipitated with zirconium in advance to synthesize an
addition-type hydroxide of rare earth metals other than ceria, followed
by adding, neutralizing and precipitating cerium ions to demonstrate the
effects of enhancing the cerium ratio in the surface portion of the
powder and highly dispersing the cerium of the surface layer.

2. Zirconia-Based Mixed Oxide Production Process

[0041](Step 1)

[0042]First, in the present invention, a zirconium salt and a salt of a
rare earth metal other than cerium are mixed in a solvent to obtain a
solution containing zirconium and a rare earth other than cerium.

[0043]There are no particular limitations on the zirconium salt.

[0044]Examples of the zirconium salt which that can be used include basic
zirconium sulfate, zirconium oxynitrate, zirconium oxychloride and
zirconium nitrate.

[0045]In the present invention, basic zirconium sulfate is used preferably
for the reason of being suitable for commercial production.

[0046]There are no particular limitations on the basic zirconium sulfate,
and examples include hydrates of compound represented by, for example,
ZrOSO.sub.4.ZrO.sub.2, 5ZrO.sub.2.3SO.sub.3 and 7ZrO.sub.2.3SO.sub.3 or
the like. One type or two or more types thereof can be used.

[0047]Furthermore, basic zirconium sulfate can be easily prepared by
mixing a zirconium salt solution (such as ZrOCl.sub.2) and a sulfating
agent (such as Na.sub.2SO.sub.4, H.sub.2SO.sub.4 or
(NH.sub.4).sub.2SO.sub.4) and heating to not less than 65.degree. C. but
less than 80.degree. C. followed by holding (aging) for a predetermined
amount of time.

[0048]The sulfating agent is added such that the weight ratio of the
sulfate radical (SO.sub.4.sup.2-) to ZrO.sub.2 is preferably 0.4 to 0.6,
and the free acid concentration of the mixture is preferably 0.2 to 2.2 N
(normal). Examples of free acids include sulfuric acid, nitric acid and
hydrochloric acid, and although there are no particular limitations
thereon, hydrochloric acid is preferably from the standpoint of having
superior productivity on an industrial scale.

[0049]Next, examples of rare earth metals other than cerium include
sulfates and chlorides of lanthanoid elements such as scandium, yttrium,
lanthanum, praseodymium and neodymium. Among these, at least one of
either a lanthanum salt or a neodymium salt is preferably contained,
while at least a lanthanum salt and a neodymium salt are particularly
preferably contained.

[0050]On the other hand, although there are no particular limitations on
the concentrations of the zirconium salt and salt of a rare earth metal
other than cerium in the mixture containing a zirconium salt and salt of
a rare earth metal other than ceria, the concentration of the zirconium
salt is 5 to 25% by weight of ZrO.sub.2 equivalent, while the
concentration of the salt of a rare earth metal other than ceria is 5 to
25% by weight of Re.sub.2O.sub.3 equivalent (wherein Re represents a rare
earth metal other than cerium).

[0051]In this manner, a solution containing a zirconium salt and a salt of
a rare earth metal other than cerium is produced in step 1.

[0052]In this step, water (pure water or ion exchange water, in
particular) can be preferably used as a solvent.

[0053](Step 2)

[0054]Next, an alkali is added to the solution containing zirconium and a
rare earth other than cerium produced in step 1 to obtain a mixed
hydroxide containing zirconium and a rare earth other than cerium.

[0055]There are no particular limitations on the alkali, and examples of
alkali that can be used include ammonium hydroxide, ammonium bicarbonate,
sodium hydroxide and potassium hydroxide. Among these, sodium hydroxide
is used preferably for the reason that it can be used inexpensively and
industrially.

[0056]There are no particular limitations on the amount of alkali added
provided it allows the formation of a precipitate from the
above-mentioned solution, and the pH of the solution is normally made to
be 11 or higher and preferably 12 or higher.

[0057]Furthermore, following completion of the neutralization reaction,
the solution containing a mixed hydroxide containing zirconium and a rare
earth other than cerium preferably is held for 1 hour or more at 35 to
60.degree. C. from the viewpoint of facilitating aging and filtration of
the resulting precipitate.

[0058]The formed precipitate composed of a mixed hydroxide containing
zirconium and a rare earth other than cerium is then recovered by a
solid-liquid separation method. The solid-liquid separation may carried
out in accordance with a known method such as filtration, centrifugal
separation or decantation. Following recovery, the mixed hydroxide
containing zirconium and a rare earth metal other than cerium is
preferably washed as necessary to remove adhered impurities.

[0059]Furthermore, although the resulting mixed hydroxide may also be
dried as necessary, in the present invention, it is normally not required
to be dried since it is used in the subsequent step.

[0060](Step 3)

[0061]In step 3, the mixed hydroxide containing zirconium and a rare earth
metal other than cerium is dispersed in water to obtain a slurry followed
by adding a cerium salt to the slurry.

[0062]Although there are no particular limitations on the slurry
concentration, it is normally 5 to 25% by weight as oxide
(ZrO.sub.2+Re.sub.2O.sub.3).

[0063]Although examples of the cerium salt include hydrochlorides,
nitrates and sulfates, hydrochlorides are preferable from the standpoint
of having superior productivity on an industrial scale.

[0064]Although there are no particular limitations on the concentration of
the cerium salt, and is 5 to 25% by weight of oxide (CeO.sub.2)
equivalent.

[0065](Step 4)

[0066]In step 4, an alkali is added to the slurry containing a cerium salt
produced in step 3 to obtain a mixed hydroxide containing zirconium, a
rare earth metal other than cerium and cerium.

[0067]There are no particular limitations on the alkali, and examples of
alkalis that can be used include ammonia, ammonium bicarbonate, sodium
hydroxide and potassium hydroxide. Among these, ammonia is preferable for
the reason of being able to be used inexpensively and industrially.

[0068]There are no particular limitations on the amount of alkali added
provided it allows the formation of a precipitate from the
above-mentioned solution, and the pH of the solution is normally 9 or
higher and preferably 10 or higher.

[0069]The formed precipitate composed of the mixed hydroxide containing
zirconium, a rare earth metal other than cerium and cerium is recovered
using a solid-liquid separation method. The solid-liquid separation is
carried out in accordance with a known method such as filtration,
centrifugal separation or decantation. Following recovery, the mixed
hydroxide containing zirconium, a rare earth metal other than cerium and
cerium is preferably washed as necessary to remove adhered impurities.

[0070]Furthermore, the resulting mixed hydroxide may also be further dried
as necessary. The drying method may be any known drying method such as
air drying or hot air drying. In addition, grinding or classification
treatment and so on may also be carried out as necessary following drying
treatment.

[0071](Step 5)

[0072]Finally, a mixed oxide containing zirconia, a rare earth metal oxide
other than ceria and ceria is obtained by heat treating the mixed
hydroxide containing zirconium, a rare earth metal other than cerium and
cerium.

[0073]Although there are no particular limitations on the heat treatment
temperature, heat treatment is normally carried out for 1 to 5 hr at
about 400 to 900.degree. C. As a result of this treatment, a mixed oxide
can be obtained that contains zirconia, a rare earth metal oxide other
than ceria and ceria.

[0074]Although there are no particular limitations on the heat treatment
atmosphere, heat treatment is normally carried out in air or an oxidizing
atmosphere.

[0075]Furthermore, the mixed oxide obtained in this manner can be crushed
as necessary. There are no particular limitations on this crushing, and
crushing can be carried out with a crushing machine such as a planetary
mill, ball mill or jet mill.

EXAMPLES

[0076]The following provides a further explanation of the characteristics
of the present invention by indicating examples thereof. Furthermore, the
present invention is not limited to these examples.

[0077]Each of the physical properties was measured using the methods
indicated below in the examples.

(1) Specific Surface Area

[0078]Specific surface area was measured according to the BET method using
a specific surface area measuring instrument (Flowsorb II, Micromeritics
Corp.).

[0080]More specifically, 0.3 g of powder were sufficiently oxidized by
heating to 600.degree. C. and holding for 60 minutes in highly pure
oxygen gas. Next, the powder was heated from 100.degree. C. to
900.degree. C. at a heating rate of 10.degree. C./min in a 5%
hydrogen-argon gas flow (100 sccm), and the hydrogen consumed during this
time was measured continuously with a quadrupole mass spectrometer to
obtain a water vapor generation curve accompanying the rise in
temperature. The amount of oxygen released was then determined from the
resulting hydrogen consumption curve and the area thereof.

[0081]Furthermore, the ceria reduction rate was determined from the
following equation.

Reduction rate=((OSC:mol-O.sub.2)/0.25 mol/mol-CeO.sub.2).times.100

Example 1

[0082]10% lanthanum nitrate (9 g of La.sub.2O.sub.3 equivalent) and 10%
neodymium nitrate (16 g of Nd.sub.2O.sub.3 equivalent) were added to a
slurry of basic zirconium sulfate (70 g of ZrO.sub.2 equivalent) followed
by the addition of 400 g of 25% sodium hydroxide.

[0083]Subsequently, the mixture was filtered and washed with water to
obtain an La--Nd-added Zr hydroxide. This hydroxide was then dispersed in
water so that the oxide was present at 5% to obtain a slurry. 10% cerium
nitrate (5 g as CeO.sub.2) was added to this slurry after which the
slurry was neutralized using 200 g of 25% ammonia followed by filtering
and washing with water to obtain a hydroxide. The resulting hydroxide was
fired for 5 hours at 650.degree. C. in air to obtain an oxide.

[0084]The specific surface area of this oxide was measured after firing
for 3 hours at 1000.degree. C. and for 3 hours at 1100.degree. C. in air.
In addition, OSC was measured together with calculating the reduction
rate.

[0085]Those results are shown in Table 1 along with the analysis values.

Example 2

[0086]10% lanthanum nitrate (9 g of La.sub.2O.sub.3 equivalent) and 10%
neodymium nitrate (11 g of Nd.sub.2O.sub.3 equivalent) were added to a
slurry of basic zirconium sulfate (70 g of ZrO.sub.2 equivalent) followed
by the addition of 400 g of 25% sodium hydroxide.

[0087]Subsequently, the mixture was filtered and washed with water to
obtain an La--Nd-added Zr hydroxide. This hydroxide was then dispersed in
water so that the oxide was present at 5% to obtain a slurry. 10% cerium
nitrate (10 g of CeO.sub.2 equivalent) was added to this slurry after
which the slurry was neutralized using 200 g of 25% ammonia followed by
filtering and washing with water to obtain a hydroxide. The resulting
hydroxide was fired for 5 hours at 650.degree. C. in air to obtain an
oxide.

[0088]This oxide was measured in the same manner as the example 1. Those
results are shown in Table 1 along with the analysis values.

Comparative Example

[0089]A mixed solution was prepared to which had been added zirconium
nitrate (88 g of ZrO.sub.2 equivalent), 10% cerium nitrate (5 g of
CeO.sub.2 equivalent), 10% lanthanum nitrate (2 g of La.sub.2O.sub.3
equivalent) and 10% neodymium nitrate (5 g of Nd.sub.2O.sub.3
equivalent). After adding 500 g of 25% ammonium to this mixed solution,
filtering and washing were carried out to obtain a hydroxide. The
resulting hydroxide was fired for 5 hours at 650.degree. C. in air to
obtain an oxide.

[0090]This oxide was measured in the same manner as in the example 1.

[0091]Those results are shown in Table 1 along with the analysis values.

[0092]According to Table 1, the articles of the present invention of
Examples 1 and 2 demonstrated a specific surface area after heating for 3
hours a 1000.degree. C. of about 55 mg.sup.2/g, a specific surface area
after heating for 3 hours at 1100.degree. C. of about 22 m.sup.2/g, OSC
of 0.21 to 0.22 mol-O.sub.2/mol-CeO.sub.2, and a ceria reduction rate of
84 to 88%. Thus, in comparison with the comparative example, the articles
of the present invention can be seen to be extremely superior with
respect to heat resistance of specific surface area at high temperatures
and ceria reduction rate.